With about 150 new heat flow values, more than 200 values of heat flow are now available from the crystalline terranes of southern California, the Basin and Range Province of Arizona, and Paleozoic sedimentary rocks of the southwestern Colorado Plateau (CP). Heat flow ranges from about 5 mW m-2 on the CP near Flagstaff, Arizona, to more than 150 mW m-2 in the crystalline rocks bordering the Salton Trough in SE California. The heat flow pattern within this region is complex, although it correlates with regional physiographic and tectonic features. Unlike the adjacent Sierra Nevada Batholith where heat flow is a linear function of near-surface radiogenic heat production, no statistically significant correlation exists between heat flow and heat production in the study area, possibly because of its complex tectonic history, involving lateral movement of basement terranes, and relatively young heat sources and sinks of different strengths, ages, and durations. Contemporary and Neogene extensional tectonism appears to be responsible for the very high heat flow (>100 mW m-2) associated with the Salton Trough and its neighboring ranges, the Death Valley fault zone and its southward extension along the eastern boundary of the Mojave block, and zones of shallow depth (<10 km) to the Curie isotherm (as inferred from aeromagnetic data) in west central Arizona. Low (<60 mW m-2) heat flow in the Peninsular Ranges and eastern Transverse Ranges of California may be caused by downward advection associated with subduction and compressional tectonics. Relatively low heat flow (67¿4 mW m-2) is also associated with the main trend of metamorphic core complexes in Arizona, and the outcropping rocks in the core complexes have a low radioactive heat production (1.3¿0.3 μW m-3) compared to the other crystalline rocks in the region (2.1¿0.2 μW m-3). ¿ American Geophysical Union 1994